1. Field of the Invention
The present invention relates to a photoelectric conversion element such as a solar cell or a sensor, having a plurality of semiconductor junctions in order to increase the conversion efficiency and having a protection member covering a photoelectric conversion section for protection during long-term outdoor use and suppression of photo-deterioration, and to a building material and a power generation apparatus using the same.
2. Related Background Art
A variety of photoelectric conversion elements have been used heretofore as independent power supplies for electrical machinery and apparatuses or as alternative energy sources. However, the cost per unit of generated energy by such photoelectric conversion elements is still high, particularly for use as an alternative power system, and research and development is now active thereon.
For example, as to materials for the photoelectric conversion section itself, there exist crystalline materials such as single-crystal silicon or polycrystalline silicon and so-called thin-film materials using amorphous silicon or compound semiconductors.
There is technology aimed to increase the conversion efficiency via the layer structure of the photoelectric conversion section consisting of a plurality of semiconductor junctions, for example, as described in U.S. Pat. No. 5,298,086.
One of the techniques for producing photoelectric conversion elements at low cost is a thin-film semiconductor production technique using a rolled stainless steel substrate and continuously forming a thin semiconductor layer thereon by use of microwave which increases the deposition rate.
A further technique is one that effectively utilizes light by providing an anti-reflection layer also serving as an electrode on the semiconductor junction layer.
In addition to these, various techniques are necessary not only for the photoelectric conversion section, but also for matters of product forms and durability and a method for connection to the system power, which are significant in practical use. Thus, research and development thereof is under way.
Particularly, solar cells are required to be designed so as to endure long-term outdoor use. Therefore, in addition to the glass protection member normally used, there are also proposals on protective members having a lightweight and flexible configuration, for example as described in Japanese Laid-open Patent Application No. 8-139347, in which the outermost surface is a transparent thin film of a fluoride polymer such as a fluororesin film and in which an encapsulating resin selected from a variety of thermoplastic, transparent, organic resins is provided inside thereof.
It is also known generally that this protection member can also have the anti-reflection effect by setting the index of refraction of the protection member between those of a transparent, resistive layer and the atmosphere.
So-called stacked device technology refers to connecting plural semiconductor junctions in series in order to increase the conversion efficiency, as described above. Generally speaking, when a configuration of two semiconductor junctions with an equal band gap is compared with a configuration of one semiconductor junction having a thickness equal to the overall thickness of the two junctions, the conversion efficiency of the double junction configuration is greater, because transit lengths of carriers are shorter and thus recombination thereof is less. In addition, the electromotive force increases while output current decreases. Therefore, the internal resistance loss due to the current decreases, so that the conversion efficiency increases. Still higher conversion efficiency can be achieved by forming plural semiconductor junctions of different band gaps and thus utilizing the light in a wide wavelength region.
However, when plural semiconductor junctions are used, how to determine conditions of each semiconductor junction becomes complicated and finding the most efficient configuration is not always easy. Particularly complex are configurations of three or more semiconductor junctions. Finding guidance and increasing freedom of design are important issues.
While such techniques to increase conversion efficiency have been researched, the most significant point in terms of the utilization of a photoelectric conversion element is that the total performance including amounts of materials used, costs for production, installation area, external view, and so on is commensurate with the resultant energy. It is thus noted that the conversion. efficiency from light to electricity is not always most important. This is the reason why attention is being paid to amorphous-based photoelectric conversion elements which can be made at considerably lower cost though having somewhat smaller conversion efficiency than the crystal-based photoelectric conversion elements exhibiting high conversion efficiency.
The amorphous silicon based semiconductors will experience the photo-deterioration phenomenon discovered by D. L. Staebler and C. R. Wronski (Applied Physics Letters, Vol. 31, No. 4, Aug. 15, 1977, p 292). This photo-deterioration phenomenon has yet to be overcome, and it is thus a significant matter to determine optimum configuration, taking account of the balance among conversion efficiencies including photo-deterioration after long-term use, as well as the initial conversion efficiency, the cost, and the like.